1. Ceramic Blank
As important characteristics of ceramic blank applied to sanitary ware, the strength of materials, thermal shock resistance and corrosion resistance can be mentioned. In addition, because ceramic products are produced by treating a powdered material to a state corresponding to its own compaction method, compacting it with the compaction method fit for the shape of a product and thereafter firing it, the characteristics of a material exhibited during the process from the material charging step to the firing step also become important, as well as the aforementioned characteristics. The important characteristics during this process include the strength at the compacting stage (referred to as wet strength in this specification because a compaction assistant such as water is generally used in the compaction of ceramics), the strength of materials after the completion of drying treatment after the compaction (referred to as dry strength), the shrinkage during the drying (referred to as dry shrinkage) and the shrinkage during the firing (referred to as firing shrinkage), and the deformation due to softening of materials during the firing (firing deformation). Among these characteristics, corrosion resistance is not particularly at issue in general applications of ceramic materials since ceramic materials fired at high temperatures have an excellent corrosion resistance.
These characteristics of ceramic materials have a plus or minus correlation between individual pairs and therefore a technical difficulty is recognized in improving all characteristics, so that there are the following problems according to the relevant art practices.
(1) Reinforcing method and thermal shock resistance of materials
The strength of ceramic blank varies with different species and a blank made using pottery stones, feldspar and clay as chief materials and densely sintering them (hereinafter, referred to as vitreous blank) has a bending strength on the order of 40-80 MPa.
Such a vitreous blank comprises a crystal phase and a glass phase, the crystal phase contains quartz and mullite. Quartz in the crystal phase is originally present in raw materials and mullite is deposited from SiO.sub.2 and Al.sub.2 O.sub.3 components, mainly of aluminum silicate minerals, in the firing process. And, the glass phase is made of silicate glass mainly comprising SiO.sub.2 and containing alkali or alkaline earth oxides.
On the other hand, in recent years, a high-strength ceramic blank in which quartz in the blank is replaced with corundum to enhance the strength has been used in place of vitreous blank. Such blank is called alumina porcelain and applied to tableware, insulators or the like.
For example, applications disclosed in Japanese patent publication Nos. 41-14914, 43-19866, 2-40015and7-68061and Japanese laid-open patent publication No. 6-232970 are known.
The ceramic blank disclosed in Japanese patent publication No. 41-14914 has a crystal phase ratio of 35-75% with the crystal phase comprising cristobalite, quartz and mullite, that disclosed in Japanese Patent publication No. 43-19866 is made by adding cristobalite, alumina and mullite to the crystal phase of an ordinary blank, those disclosed in Japanese patent publication Nos. 2-40015 and 7-68061 have a crystal ratio of 40% or more with the crystal phase containing corundum, mullite, cristobalite and quartz, and all of them intend to promote the ceramic strength and reduce the firing deformation. In addition, Japanese laid-open patent publication No. 6-232970 discloses application of alumina porcelain to sanitary ware.
The bending strength of alumina porcelain is 150-300 MPa, so that reinforcement of not less than twice the strength of vitreous blank is possible. The principle of this reinforcement is considered as follows.
With the vitreous blank, a large difference in thermal expansion coefficient between the quartz and the glass phases in the blank causes minute cracks (microcracks) to occur around quartz due to strain during the cooling process in the firing. The presence of these microcracks lowers the original strength.
By contraries, with the alumina porcelain, since quartz is replaced with corundum, the amount of microcracks decreases and the strength is enhanced.
In addition, because uniformly dispersing more minute and stronger corundum particles than quartz in the blank prevents cracks from occurring due to stress breakage from proceeding, the strength is further enhanced.
Such alumina porcelain has an excellent characteristic in strength, but when utilized in the same uses with the vitreous blank, the following points become at issue.
Though containing numbers of microcracks present in it as mentioned above, the vitreous blank is excellent from the standpoint of thermal shock resistance since these microcracks relax the stress occurring due to thermal shock.
On the other hand, with a conventional alumina porcelain, since microcracks decrease in number as mentioned above, thermal shock resistance lowers and in particular with large articles such sanitary ware, there is a problem that articles may break during the cooling process in the firing.
In addition, with a conventional alumina porcelain, since the added amount of alumina is increased to reinforce the blank, the ratio of crystal phase in this blank is higher than in the vitreous blank. With a higher crystal phase ratio, however, the molten amount of raw material particles decreases which changes in the glass phase serving to forward the sintering, so that there is a worsening tendency of sintering.
For this reason, sintering at higher firing temperatures has been considered for use with the conventional alumima porcelain as compared to the vitreous blank, but sintering at a lower temperature is desirable from the viewpoint of firing cost, and because the same firing temperature as with the vitreous blank also has an advantage that firing can be performed at the same production facility as the vitreous blank.
On the other hand, to sinter the alumina porcelain blank at a lower temperature, a method for increasing the content of Na.sub.2 O or K.sub.2 O serving as a sintering assistant to accelerate the vitrification of raw materials, and a method for further pulverizing the powder of raw materials are also carried out.
In the case of increasing the content of a sintering assistant, however, since the viscosity of vitrified raw materials in the blank during the firing decreases, the firing deformed amount of the blank due to stress such as dead weight during the firing is forced to increase, so that the deformed amount at the production stage of products increases.
As mentioned above, conventional alumina porcelain ceramics have problems of being poor firstly in thermal shock resistance and secondly in sintering.
With respect to sintering problems, there is a method for increasing the content of a sintering assistant or a method for further pulverizing the powder of raw materials, but a new problem of increase in the firing deformed amount of the blank takes place alternatively as discussed.
(2) Reduction method of firing deformed amount and matching of glaze.
Ceramic blank is applied to various products. For civil life, tableware and sanitary ware are mainly used and insulator set and such others are used to commercial purposes, but generally in these products, ceramic blank alone is not employed and glaze is applied to the surface thereof from the viewpoint of ornament or function for use.
As glaze, for example, Bristle glaze is mainly employed for sanitary ware. This Bristle glaze, mainly comprising RO.sub.2 (acidic oxide with a main constituent of SiO.sub.2), R.sub.2 O.sub.2 (amphoteric oxide with a main constituent of Al.sub.2 O.sub.3) or R.sub.2 O+RO (basic oxide mainly constituent of K.sub.2 O, Na.sub.2 O, Ca.sub.2 O, Zn.sub.2 O, MgO, BaO and SrO) and if necessary containing emulsifier such as zirconia and tin oxide and various stains, is applied to a dried blank and is fired at the same temperature as the blank.
And as mentioned above, if raw material powder is further pulverized to reduce the firing deformed amount, the deformation during the firing decreases, but application of glaze such as Bristle glaze to the surface of blank would lead to an increase in the shrinkage of blank at the initiating temperature for melting of glaze in the firing process as compared with the raw material powder material which is not further pulverized, so that there is a problem that rivelling or peeling in the glaze is generated due to the resultant distortion. Alternatively, if sintering assistant is increased in quantity, the viscosity of vitrified raw materials decreases and sintering proceeds rapidly, so that a similar problem takes place.
(3) Sintering density of materials and strength of blank.
Generally, ceramic materials improve in strength with denser sintering. However, with ceramic materials partly or abundantly containing glaze like subjects of the present disclosure, the firing deformation due to the softening of materials increases with the progress of sintering in materials. Accordingly, with ceramic materials partly or abundantly containing glaze, if the firing shrunk amount or firing deformed amount of materials is reduced by limiting the sintering density to a low extent, the strength of materials lowers as a result.
(4) Wet strength, dry strength and dry shrinkage of materials.
Wet strength, dry strength and dry shrunk amount of materials are very important characteristics in the production of products. Particularly in the production of large-sized products such as sanitary ware, high wet strength and dry strength are required. In addition, the dry shrunk amount is preferably smaller to purposes of preventing any tearing in the drying step. With conventional ceramics, these characteristics are yet insufficient and from the viewpoint of strength for breakage or the like of products during handling in the production step, the thickness of materials is forced to be increased to a greater extent than necessary. Furthermore, because of a large dry shrunk amount, a gradual drying is required to prevent the occurrence of tearing, so that there is a problem of taking much time in drying.
2. Sanitary ware
(1) Sanitary ware is heavy and correspondingly has a personal burden for producers, distributors, constructors and such others who handle same.
This is because characteristics of blank are a primary factor and from the relation between the deformation due to the softening of blank during the firing and the strength of blank, it is required to keep the blank thickness equal to or larger than a predetermined thickness, thereby leading to a heavy weight of products. On the other hand, it is difficult to make the blank thickness of sanitary ware thinner for a lighter weight. When the blank thickness decreases, the deformed amount of blank during the firing increases nearly at the inverse proportion to the square of blank thickness, whereas the strength decreases at the direct proportion to the square of blank thickness, so that a reduced blank thickness will lead to an increase in the deformation and a radical fall in the strength of products. As an alternative method, it is considered to achieve a reduction in weight by utilizing a blank of small specific weight, but there is a problem that a blank of small specific weight is generally low in strength.
(2) Dimensional accuracy is poor and the production of large-sized articles is difficult. The degree of freedom in design is low when applied to sanitary ware.
Because of being made of ceramic blank, sanitary ware is generally low in the degree of freedom in design, poor in dimensional accuracy and presents difficulties in the production of large-sized products and consequently has many problems associated therewith. Especially because the demand of consumers for design is elevated, an improvement in the design of sanitary ware has become a great problem.
In addition, sanitary ware is conventionally molded by slurry slip molding and fired after the molded products have been dried. Deformation due to the shrinkage and softening of blank during the firing in this production process forms a principal factor for worsening the degree of freedom in design and dimensional accuracy and hindering the production of large-sized articles. With conventional sanitary ware, a large firing shrinkage and deformation present a problem in achieving the promotion of an increase in the degree of freedom in design or the like. On the other hand, if a reduction in firing shrinkage or deformation is intended, a decrease in the strength of blank presents a problem and an increase in blank thickness for the prevention of a decrease in the strength of products will cause a problem of an increase in weight.